The first probing of ultrasonic fields in liquid involved in the use of miniature hydrophones consisting typically of a small crystalline or ceramic piezoelectrically active element, which was mounted together with suitable backing at the end of a tube or needle or other similar supporting structure. Despite their small size, such hydropohones unavoidably altered the acoustic field at the probed point because of the large difference in acoustic impedance between the hydrophone materials and the liquid medium in which the hydrophone was immersed during use. Furthermore, material and geometric factors of the sensing element and supporting structure led to multimode response, undesirable reflections and a complicated frequency and angle dependence of the response.
These problems were overcome to a great extent by replacing the ceramic active element with the piezoelectric polymer polyvinylidene fluoride (PVDF). However, in the needle-like geometry, non-uniformities in the frequency response still occurred because of the presence of the hydrophone housing.
To further improve the performance of polymer hydrophones, the spot-poled membrane design was developed, as shown in U.S. Pat. No. 4,433,400. In this design a single sheet of polymer film takes the form of a flat membrane held taut by means of a hoop or other convenient supporting structure which is made sufficiently large so as to remain, during use, outside the region of the medium sustaining acoustic wave fields and adequately far away from the field point probed. Typically the electrodes have the form of circular spots and the electrical leads have the form of fine lines. Multiple-element arrays also are possible. In cases when the acoustic field is confined within a collimated beam, the probe is oriented so that the membrane is perpendicular to the beam. Because the supporting structure is outside the beam, there are no significant reflections, and there is no significant response from unwanted modes. In particular, the response to normally incident plane wave fields is essentially independent of frequency below the thickness resonance frequency of the polymer membrane, which makes this design useful as a standard hydrophone against which the frequency response of other hydrophones can be compared.
Although this design is better than previous ones, it is not without certain problems and limitations. In particular, it suffers from the following deficiences:
a. Because the leads on the membrane are exposed, the hydrophone cannot be used to probe acoustic fields in electrically conductive fluids. For example, use in isotonic saline or similar biological fluids is prohibited because of the electrical shunting effect of these fluids.
b. The hydrophone sensitivity is dependent on the dielectric constant of the ultrasound propagation liquid surrounding the hydrophone. Thus no single hydrophone calibration factor can be specified; the probe must be recalibrated each time the dielectric properties of the surrounding liquid change.
c. For use in water or acoustically water-like media (the most common application), the large relative dielectric constant of the water (=80) surrounding the membrane lead causes the following problems:
(i) Significant reduction in hydrophone sensitivity occurs because of the electrical capacitance loading effect of the underwater leads.
(ii) The increased lead capacitances causes degradation of performance (i.e., increased noise voltage and decreased frequency response) when charge amplifiers are used to amplify the hydrophone signal.
(iii) The increased lead capacitance, as well as the mutual capacitance between elements, increases electrical crosstalk when multiple element hydrophone arrays are used.
d. Signal-to-noise level is degraded because of the susceptibility of the exposed electrical leads to radio frequency interference (RFI) pick-up.
e. The hydrophone directional response pattern can have an undesirably large side lobe structure due to the fact that the membrane geometry is capable of supporting surface waves.
f. The exposed electrical leads on the membrane are unprotected. This makes them highly susceptible to damage, which limits their usefulness outside of a laboratory setting and discourages commercial development and promotion of the hydrophone.